12.3. Blast Resistant Structural Design

A study of 17 British buildings hit by German bombs during World War II examined eight steel-framed buildings, five reinforced concrete buildings and four wall-bearing buildings. The steel-framed buildings included office, apartment and industrial buildings, and a two-story railway station.

The weight of bombs ranged from 110 lb to 3,100 lb. In each case the charge weight was approximately 50 percent of the bomb weight. With one exception all were internal explosions and the type of damage was fairly typical. One example is the explosion damage to a seven-story apartment building. This building consisted of a concrete-encased steel frame (for fire protection). The floors were 6” hollow tile with 3.5” concrete topping, supported on steel beams and girders. Exterior walls consisted of 9” brick and tile facing. Interior walls were 3” brick with plaster surfaces.

A 1,100-lb bomb perforated the roof and three floors and detonated just above the fourth floor. Damage to the seventh floor consisted of a failed girder due to impact from the bomb and about 100 sq. ft of floor area removed. Damage to the sixth floor included a buckled girder with torn out connections, several deflected beams and approximately 190 sq. ft of floor area removed. On the fifth floor, one girder deflected about 7”. Several other floor beams were bowed. Approximately 650 sq. ft of floor area was demolished. On the fourth floor, one girder was blown down together with four beams. One column deflected 7” and twisted, and about 700 sq. ft of floor area was demolished. The fifth floor was blown up; the fourth was blown down. One bay on each of the first, second and third floors is believed to have collapsed due to weight of debris from above. There was no fire. Due to the sufficient redundancy of the steel frame, the building did not collapse.

Another example of a steel-framed building subjected to an internal explosion was the World Trade Center on Feb. 26, 1993. A van containing approximately 1,800 lb. of fertilizer-based explosives was parked on an exit ramp just south of column 324, one of the main steel columns supporting the 110-story tower structure. The column measured about 4’ by 4’ across. It and six adjacent columns lost their fireproofing and lateral restraint (the bracing provided by the concrete floors that were blown out around them), but otherwise were not damaged by the explosion. The fact that the column did not buckle from the significant increase in its effective length speaks well for the redundancy in a building that probably was not designed for blast loading.

For more information see Reference 1.

[1] “Structures to Resist the Effects of Accidental Explosions,” Dept. of the Army Tech. Manual, TM5-1300, Dept. of the Navy Pub. NAVFAC P-397, Dept. of the Air Force Manual, AFM 88-22, June 1969

The first difference is in the way a given structure is loaded. In the case of an earthquake the structure is subject to ground motions that shake the structure from the base up. In the case of an explosion produced by an air or a surface burst, the structure is loaded by means of a compression wave (shock wave) over some area. Since a portion of the blast energy is coupled into the ground, the structure is also subject to ground motions similar to an earthquake, though much less intense.

A second difference is the duration of loading. For earthquakes, the duration of induced motions (shaking) can range from seconds to minutes. Additional loadings are produced by “aftershocks,” which are generally less intense than the initial shaking. For conventional explosives, the duration of a pressure wave is on the order of milliseconds.

For example, in the Oklahoma City event, the yield of the weapon was approximately 4,000 lb TNT equivalent. The truck containing the explosive was positioned about 10’ from the building. The peak pressure at the face of the buildings was about 1,900 psi, and the duration of the positive phase of the pulse was approximately 3 ms. Judging by the size of the crater, a fair portion of the energy coupled into the ground, producing ground shock. However, judging by the damage, clearly air blast was the primary damage mechanism. Further, earthquakes shake an entire building, but produce mostly horizontal loads at floor-slab levels, concentrating in the specially designed, laterally stiffer structural systems. Blast usually does not attack the entire structure uniformly, but produces the most severe loads to the nearest structural elements, both vertical and horizontal, with little regard to their stiffness. Uplift pressure load on floors is also a specific blast effect.

The term ductility refers to the ability of the material to absorb energy inelastically without failure—the greater the ductility, the greater the resistance to failure. Blast-resistant designs often conservatively assume elastic response in order to simplify design, minimize permanent (plastic) deformations, and reduce post-blast repairs, especially where functional continuity of the facility is considered. Due to their highly ductile features, structural steel frames provide additional ultimate resistance for a blast event exceeding in severity the design blast.

Ductile inelastic structural response can be expected during both severe blast and severe earthquake events. However, it is generally recognized that plastic hinge zones and ductility demands in the two events do not necessarily match because of the differences in the loading patterns and effects.

Yes. The inertia, as measured by the mass of the structure or structural member, is an important factor in the response to a dynamic-impulse lateral load such as a shock wave. Because steel is the most dense construction material, heavy and robust steel members are especially effective in resisting blast loads. This is evident in the performance of heavy tanks and battleships, the ultimate blast-resistant structures.

For ordinary buildings, like apartments, offices, and stores, building codes do not require blast resistance. For buildings that house hazardous processes, building codes require special safety considerations.

The lateral stability of a moment frame is dependent on the bending stiffness of rigidly connected beams and columns. Adequate diagonal bracing or shear walls at selected locations provide the lateral stability of a braced frame. Elements of lateral stability often are distributed more uniformly in moment frames, in which case each part of the building is more likely to be stable on its own. Therefore, moment frames are the better choice for blast-resistant design. In braced frames, the diagonal braces or shear walls can be knocked out by an engulfing blast wave, reducing the effectiveness of the braced frame, unless special features are included to mitigate this potential behavior.